The study of toxic heavy metals in the environment is challenging because metals are a natural component of our environment. As such, they have disfinct biogeochemical cycles that govern their mobility, fate, and toxicity. A failure to understand these biogeochemical cycles will lead to a failure to adequately assess adverse impacts. Current risk assessment techniques for metals via the oral ingesfion route are based on operationally defined tests which mimic the release of metals from soil in the human digestive system. A goal of this proposal is to be able to define the bioaccessibility of metals in geochemical terms, not based on operafionally defined extraction methods. These goals will be tested through a combination of laboratory and in situ studies of metal-contaminated soils and mining wastes to characterize the interplay of processes that increase metal bioavailability over time and those that decrease metal bioavailability over time. Through these experiments, we will test whether the bioaccessibility of metals through oral ingestion can be predicted from the underiying speciation of metals and the characterisfics ofthe soil itself. This study will bridge the current gap between geology and toxicology. Current risk assessment methods can determine if metals in a soil may exert risk, but not why. It is our intent to show that through the incorporafion of a modest amount of geochemical analysis, we can greatly improve risk assessment for metals in soils by showing why some soils exert risk while other soils with similar levels of metals do not exert risk. In addifion, with a similarly modest understanding of the temporal cycling of metals, we can show that bioaccessibility of metals in soils is not fime invariant. With this knowledge, risk assessment can be proactive in understanding the nature of both current and future risks.